The purpose of this study is to investigate whether the marketed drug, losartan, is safe and effective in the treatment of diabetic kidney disease. Losartan has been approved for the treatment of hypertension, and is currently being studied in the treatment of congestive heart failure. The use of losartan for the treatment of diabetic kidney disease is investigational. It is thought that losartan might be effective in preserving kidney function based on some animal studies that suggest that losartan can decrease the amount of "protein leaking" in the urine, and slow down the rate of decline of kidney function including in animal models of diabetes.